To keep up with the recent trend toward reduction in the size and the weight of electronic equipment, a compact capacitor showing low impedance in the high frequency region and having a large capacitance is demanded. As a capacitor for use at high frequency, mica capacitor, film capacitor, ceramic capacitor and the like have been heretofore used. These capacitors are, however, not suitable for achieving a large capacitance. As a compact capacitor having a large capacitance, aluminum electrolytic capacitor and tantalum electrolytic capacitor are generally used.
The electrolyte used in these electrolytic capacitors is a liquid electrolyte or a solid manganese dioxide. In recent years, a capacitor where a TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salt, which is an organic semiconductor, is used as the solid electrolyte has been proposed.
This capacitor is disadvantageous in that although the TCNQ complex salt is heated/melted, impregnated into an electrode and cooled/solidified to form a solid electrolyte, the TCNQ complex salt is likely to decompose and deteriorate at the melting temperature. Therefore, the production process thereof becomes very complicated and the cost increases.
In order to solve these problems, use of a solid electrolyte comprising a polymer of 5-membered heterocyclic compound having an electrical conductivity higher than the manganese dioxide or TCNQ complex salt, such as pyrroles, thiophenes and furans, have been proposed. A solid electrolytic capacitor using such an electrically conducting polymer has superior frequency properties compared with electrolytic capacitors using an electrolytic solution, because the electrically conducting polymer exhibits high electrical conductivity.
With respect to the surface treatment of electrochemically formed film (dielectric film) of the electrode, a method of allowing silicic acid or silicate to be present on the film surface to prevent deterioration in the capacitance and dielectric loss at high temperature and high humidity is known (see, JP-A-5-234821 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-5-234822). However, this method is problematic with respect to humidity resistance.
Also, a solid electrolytic capacitor using a valve-acting metal having formed thereon a dielectric layer after surface treating the valve-acting metal by impregnating it with a silane coupling agent solution is known (JP-A-2-74021). The silane coupling agent is used in the form of an aqueous solution, and silanol is produced after hydrolysis and reacts by condensation with the hydroxyl group of the dielectric film to form a covalent bond. Therefore, heating is necessary. Furthermore, a thin film capacitor which is surface treated by dipping it in a solution of chlorosilane-based surfactant containing a fluorinated carbon chain (see, JP-A-4-36721) is known. In this technique, the chemical reaction group of the silane compound chemically bonds to the hydroxyl group of the dielectric film in a non-aqueous solvent system, and surface modification can be attained. However, HCl may be side produced to damage the dielectric film, and the reagent is expensive and readily reacts with water. As a result, this method is problematic with respect to profitability and stability of the reagent.
In the case of a solid electrolytic capacitor where the dielectric film on the valve-acting metal is an inorganic material and the electrolyte formed on the film is an electrically conducting polymer of an organic material, adhesion between the electrically conducting polymer and the dielectric film is weak and separation therebetween takes place very often at high temperatures. Accordingly, the capacitance disadvantageously decreases with time.